Sagnac interferometer type current sensors operate by measuring the Faraday effect-induced phase shift of circularly polarized light waves traveling in a closed path around a current carrying wire. Fiber optic current sensors are advantageous over iron-core current transformers, since fiber optic sensors are non-conductive and light weight. Further, fiber optic sensors do not exhibit hysteresis and provide a much larger dynamic range and frequency response. An in-line and a loop version of the fiber optic interferometric sensor are described in detail in allowed U.S. patent application titled "Fiber Optic Interferometric Current and Magnetic Field Sensor," Ser. No. 08/620,121, and filed on Mar. 26, 1996 by James N. Blake, incorporated herein by reference.
However, linear birefringence in the sensing region of the fiber optic sensor may cause large scale factor errors. The linear birefringence may be introduced by imperfections in the fiber, such as internal asymmetry. Linear birefringence is also introduced by the bending of the fiber around the current-carrying conductor in the sensing region. Therefore, it is nearly impossible to eliminate the linear birefringence without annealing the fiber. Further, because the linear birefringence and therefore the scale factor error varies with temperature and other environmental variables, it is difficult to predict and correct for it with sufficient accuracy.